1919] Coit-Hodgson : Abnormal Shedding of Washington Navel Orangt 297 



of young Navel oranges occurs throughout the great interior valleys 

 of California and in districts very remote from any possible source of 

 noxious vapors, there is little possibility that the gas factor can be 

 operative in the case under consideration. 



Fungi and Bacteria as a Cause of Abscission 



Although the belief is commonly held by plant pathologists that 

 fungus parasites sometimes cause the shedding of plant parts, the 

 literature on this phase of abscission is very meager. Inoculations 

 with Bacterium citrarefaciens, the organism causing Citrus Blast, 

 carried on in our greenhouses have shown that when the organism is 

 inoculated into the tip of the young leaf the latter is shed within a 

 few days. Rolfs has reported that shedding of mature oranges fre- 

 quently occurs in Florida, due to the common wither tip fungus. 

 Colletotrichum gleosporioides. However, we are concerned here with 

 the shedding of immature fruits and it is by no means clear that the 

 process resulting in shedding is the same in both cases. 



For many years growers of Washington Navel oranges have ex- 

 perienced losses from a black rot disease of the fruit which manifests 

 itself as a stimulation of the fruit, causing it to grow to an extra large 

 size, ripen early and assume a deep red color, with a certain amount of 

 dropping. This disease was first noted by N. B. Pierce 32 in 1892 and 

 was first described by him in 1902 33 as "Black Rot of the Navel 

 Orange" caused by the fungus Alternaria citri. 



The fruit is infected when quite small, probably just before or 

 soon after the style is shed, through the cracks and imperfections in 

 the proliferations of the navel (pi. 34) . The fungus is a weak parasite 

 and remains quiescent, or nearly so, during the growing period of the 

 young fruit, at which time the fruit is more or less resistant to the en- 

 croachments of parasites. With the decline in vigor incident to approach- 

 ing maturity the fungus becomes more active and exerts a stimulating 

 influence on the fruit, causing it to take on a deep reddish-yellow color 

 and to ripen earlier than the normal fruit. In a small and restricted 

 area the cells of the pulp are broken down and become a nauseating 

 mass of black fungus mycelia and spores. The rind is left uninjured 

 until the disease has made considerable progress within, but ultimately 

 a black and decayed spot appears on the sur^pe near the navel end. 

 A certain proportion of the infected fruits early shows a yellow spot 



32 U. S. Dept. Agr. Yearbook (1892), p. 239. 

 33Bot. Gaz., vol. 33 (1902), pp. 234-35. 



